What I consider to be excessive spawn killing is when you do not give the opposing team a chance to move more than a couple of steps. By that I mean that they don't have a slightest chance of providing any kind of resistance (shooting back or ducking for cover). IED's C4 and RPG in the spawn clearly qualify, and also "exploiting" some map's possible weakness (Market for example), where you take a position and gun down people after they take a few steps. I made my bones in COD 5 and that is the rule we follow there. Don't shoot someone who can't shoot back (because they just spawned).
Now I give warnings because I can't ban people, only kick, but some people really push it. I also realize that sometimes people do gun down others really close to the spawn (myself included) because things can sometimes get heated, but when warned or just alerted to that fact, they quit doing that. But there are others on the other hand that do not abide by this rule and need a kick in the ass sometimes
That's my position